The present invention relates generally to accelerometers, and more particularly, to a variable capacitance bridge accelerometer.
It is well known that capacitive accelerometers measure the acceleration, vibration and the inclination of objects to which they are attached. These objects typically include missiles, spacecraft, airplanes and automobiles.
In general, capacitive accelerometers change electrical capacitance in response to acceleration forces and vary the output of an energized circuit. Capacitive accelerometer systems generally include sensing elements, including capacitors, oscillators, and detection circuits.
The sensing elements include at least two parallel plate capacitors functioning in differential modes. The parallel plate capacitors generally operate in sensing circuits and alter the peak voltage generated by oscillators when the attached object undergoes acceleration.
When subject to a fixed or constant acceleration, the capacitance value is also a constant, resulting in a measurement signal proportional to uniform acceleration.
This type of accelerometer can be used in an aerospace or in a portion of aircraft or spacecraft navigation or guidance systems. Accordingly, the temperature in the operating environment of the accelerometer changes over a wide range. Consequently, acceleration must be measured with a high accuracy over a wide range of temperatures and temperature gradients. This is often a difficult and inefficient process.
The disadvantages associated with current accelerometer systems have made it apparent that a new accelerometer is needed. The new accelerometer should substantially minimize temperature sensing requirements and should also improve acceleration detection accuracy. The present invention is directed to these ends.
In accordance with one aspect of the present invention, an accelerometer includes a flexure plate section and a rigid plate section enclosed within a housing. The flexure plate section includes a first fixed plate, a second fixed plate spaced apart from and in parallel relation to the first plate, and a flexure plate disposed between and in substantially parallel relation to the first and second plates. The flexure plate is coupled to the housing along at least an edge. The flexure plate and first plate define a first distance and a first capacitor and the flexure plate and the second plate define a second distance and a second capacitor, wherein the first and second distances vary in response to acceleration forces acting upon the flexure plate. The rigid plate section is enclosed within the housing and electrically isolated from the flexure plate section. The rigid plate section includes a third fixed plate, a fourth fixed plate spaced apart from and in parallel relation to the first plate, and a rigid plate disposed between and in substantially parallel relation to the first and second plates. The rigid plate is electrically isolated from the housing. The rigid plate and the third plate define a third distance and a third capacitor and the rigid plate and the fourth plate define a fourth distance and a fourth capacitor, wherein a fifth distance is defined between the rigid plate and the flexure plate such that the fifth distance is a function of the acceleration forces. The first, second, third, and fourth capacitors connect to form a bridge adapted to generate a bridge voltage signal.
In accordance with another aspect of the present invention, a method for operating an accelerometer system includes accelerating a flexure plate, thereby causing a first distance between the flexure plate and a first fixed plate to change and thereby causing a second distance between the flexure plate and a second fixed plate to change. A bridge waveform is then generated and rectified as a function of a reference excitation phase. A rectified waveform is generated and filtered. An analog signal is generated and converted to a digital signal. A digital word is generated.
One advantage of the present invention is that it generates a dynamic range and granularity sufficient for Inter-Continental Ballistic Missile (ICBM) usage. Moreover, the bridge accelerometer consumes less power than current accelerometers, while dramatically improving reliability.
Another advantage is that it is not substantially affected by changes in temperature or temperature gradients. While the bridge configuration reduces the temperature sensitivity, the AC excitation allows narrow band analog filtering. These effects enhance the signal-to-noise ratio.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.